I first learned Java in 1997 and have been using it professionally since 2005. If your Kotlin code is pretty indistinguishable from your Java code, you aren't using Kotlin properly. Here is what I wrote on this topic in 2022:
Here are some of my favorite Kotlin improvements over Java that I leverage all the time:
* Much improved type system (nullable types, function types, declaration site variance, type aliases, contracts, better type inference, reified function type arguments)
* Local variables are final by default ("val")
* Type-level distinction between read-only and mutable collections (but compiled to Java collections under the hood, so no conversion required when interacting with Java)
* Much improved collection API
* Much improved lambdas (e.g., no pain points w/ mutating variables and checked exceptions)
* Extension functions (incredibly useful in practice)
* Much better DSL capabilities (great for UIs, generating HTML, etc.)
* Lazy properties (more generally: delegated properties)
* Coroutines (looking forward to Java's Loom; by then coroutines will have dramatically improved my async code for 5+ years)
* Great serialization support (kotlinx.serialization)
* Pragmatic macro-like capabilities via inline functions and compiler plugins (removes lots of boilerplate)
* Multiplatform support (JVM/JS/WASM/native; Graal native image is a good alternative to Kotlin/native and also works for Java)
Here are some of my favorite Kotlin improvements over Java that I leverage all the time:
* Much improved type system (nullable types, function types, declaration site variance, type aliases, contracts, better type inference, reified function type arguments)
* Local variables are final by default ("val")
* Type-level distinction between read-only and mutable collections (but compiled to Java collections under the hood, so no conversion required when interacting with Java)
* Much improved collection API
* Much improved lambdas (e.g., no pain points w/ mutating variables and checked exceptions)
* Extension functions (incredibly useful in practice)
* Much better DSL capabilities (great for UIs, generating HTML, etc.)
* Lazy properties (more generally: delegated properties)
* Coroutines (looking forward to Java's Loom; by then coroutines will have dramatically improved my async code for 5+ years)
* Great serialization support (kotlinx.serialization)
* Pragmatic macro-like capabilities via inline functions and compiler plugins (removes lots of boilerplate)
* Multiplatform support (JVM/JS/WASM/native; Graal native image is a good alternative to Kotlin/native and also works for Java)